General production methods for liquid crystal display apparatus having a liquid crystal display panel comprising two transparent substrates, to which electrodes have been applied with an alignment treatment, with liquid crystal sealed therebetween include a method of attaching two substrates to each other via a spacer by the substrates alignment, compressing by a press plate, a weight, or a vacuum packing to provide a uniform cell gap, and curing by heat as disclosed in JP-A-59-232315 or JP-B-6-68669.
Among the production methods for a liquid crystal display panel, when a weight or a press plate is used to apply compression to provide a uniform cell gap, there is a problem that the stress on the substrate becomes uneven, generating peel-off of the seal material at a panel corner portion. When a vacuum packing is used to apply compression, there is a problem of dislocation of the aligned substrates caused by the shrinkage of the vacuum packing bag in applying compression.
Examples of liquid crystal seal materials used for attaching the two substrates and sealing a liquid crystal (hereinafter abbreviated as a seal material) include one-pack type thermosetting epoxy resins such as "STRUCTBOND XN-21-F" produced by MITSUI TOATSU Chemical, INC.
Such seal materials containing a thermosetting epoxy resin have problems such as dislocation of the two substrates that are preliminarily aligned and attached, generation of substrate warping due to a high curing temperature (150.degree. C.), and a long curing time.
In order to solve the problems, methods using an ultraviolet ray curing type seal material, a seal material containing both an ultraviolet ray curing type component and a thermosetting type component, and a double type seal material overlapping a thermosetting curing type seal and a ultraviolet ray curing type component can be used.
Examples of ultraviolet ray curing type seal materials include those containing (meth)acrylic acid monomer capable of curing by radical polymerization. A "(Meth)acrylic acid monomer" herein denotes a compound comprising at least one selected from the group consisting of acrylic acid monomer, acrylic acid ester monomer, methacrylic acid monomer, methacrylic acid ester monomer and derivatives thereof.
Since the curing shrinkage by the ultraviolet ray is significant and the curing rate of seals is low in the curing by radical polymerization, there are problems in fixing and of moisture resistance.
In order to prevent such curing shrinkage, use of a modified urethane acrylate is advocated in JP-A-7-13173 and JP-A-7-13174.
However, urethane acrylate expands by heat and is not heat resistant. Particularly in the annealing process conducted to ensure the uniform display plane of the liquid crystal display apparatus, since the annealing needs to be carried out at the nematic-isotropic phase transition temperature (generally from 80.degree. C. to 120.degree. C. or higher), seal materials containing urethane acrylate expand to cause problems such as generation of threshold voltage irregularity caused by the gap height in the vicinity of sealed portions of the liquid crystal panel.
Examples of seal materials containing both an ultraviolet ray curing type component and a thermosetting type component having an improved curing rate include those disclosed in JP-A-4-11223 and JP-A-6-160872. As a commercially available product, "WORLDROCK X-8700" produced by KYORITSU Chemical and Co., LTD. is known.
Since such seals containing both a ultraviolet ray curing type component and a thermosetting type component include a resin obtained by polymerizing (meth)acrylic acid monomer or oligomer as the ultraviolet ray curing component, and an epoxy resin as the thermosetting type component, the epoxy resin disturbs the radical polymerization of (meth)acrylic acid monomer or oligomer in ultraviolet ray curing to require a great amount of ultraviolet ray energy. Further, since the thermosetting component is an epoxy resin, thermosetting at a high temperature over a long time becomes necessary, which causes problems such as decline of yield in production caused by warpage of glass substrates.
Also a method of adopting a double type seal comprising a thermosetting seal material on the verge of a liquid crystal and applying a ultraviolet ray curing type seal parallely is advocated in JP-A-3-273215 and JP-A-4-11223.
Further, in a production process of a liquid crystal display panel further comprising a step of dropping and filling liquid crystal between a step of forming a seal material on substrates with electrodes applied with the alignment treatment and a step of attaching opposing substrates via a spacer means, since uncured seal material and liquid crystal contact each other, a seal material polymerized by a radical polymerization and capable of ultraviolet ray curing is required. If a monomer or an oligomer polymerized by a cation polymerization is used, since a cationic compound such as aromatic group diazonium salt of Lewis acid, triallylsulfonium salt, diallyl iodonium salt, triallyl selenium salt, or a metallocene compound is used as an optical initiator, the ion component will leak into the liquid crystal to cause problems such as poor alignment of the liquid crystal and increase of the electric current amount. Therefore, a double seal method applying an ultraviolet ray curing type seal polymerized by radical polymerization on the edge of the liquid crystal, and an epoxy seal parallel thereto is suggested in JP-A-62-89025.
Since the above mentioned double seal method requires a seal twice as wide as a usual one, it causes problems by restricting the product design of the liquid crystal panels, such as a reduced area of a display portion of a liquid display panel, and a smaller electrode terminal portions necessary for installation.